1. Field of the Invention
This invention relates to a Cr--Mo steel for which preheating and postheating treatments can be effectively omitted, and a welding method thereof, wherein said steel pipe is for application as steam piping in power generation plants etc. such as steam piping made of STPA23, as well as to on-site welding of such pipes.
2. Description of the Prior Art
In general, 1.25Cr-0.5Mo steel pipes such as STPA23 possess extremely high hardness at welding and it has thus been necessary to apply preheating and postheating treatments in order to reduce the hardness as well as prevent low temperature cracking during welding and avoid stress corrosion cracking during use of the piping. This means that a preheating treatment is necessary to prevent the occurrence of weld cracking and, on the other hand, if said steel pipes are left welded as they are, the absorbed energy at Charpy impact testing under room temperature remains at 2 to 7 kg-m and it becomes indispensable to improve it to around 10 to 15 kg-m by applying a postheating treatment at 550.degree. to 700.degree. C.
Various efforts have since been exerted to solve this problem and, for example, on page 371 of No. 2, Vol. III of The Collection of Papers of the Welding Society issued in 1985 a description is given on shielded arc-welding of a steel plate containing C, Cu, Ni, Cr and Mo within a specific range, said steel plate being first preheated to a temperature of 100.degree. C. or less to eliminate the necessity for a postheating treatment, and on multi-layer SAW welding after preheating to a temperature of 225.degree. C. or above, also maintaining the interpass temperature at 225.degree. C. or above, to eliminate the need for a postheating treatment. A similar description can be found in JP,B 61-56309.
Also, in HPIS, stress annealing criterion and explanation thereof, it is stated that when steel materials of the standard composition of SCMV3, STPA23 or STBA23 are preheated to a temperature of 150.degree. C. to 300.degree. C. and if the interpass temperature is maintained at 150.degree. C. to 300.degree. C., a postheating treatment can be omitted. A similar description can be found in JP,B 61-56309.
With a 1.25Cr-0.5Mo steel material, the thermally influenced section undergoes extreme hardening during welding and tends to cause low temperature weld cracking. In order to prevent this, it is necessary to apply a preheating treatment at 200.degree. C. to 350.degree. C. to lessen the degree of hardening of the thermally influenced section during welding and, also, to reduce the content of diffused hydrogen which leads to the occurrence of cracking. In many cases, it becomes necessary to apply a postheating treatment at 620.degree. C. to 700.degree. C., in order to soften the thermally influenced section during welding, to eliminate or reduce residue hydrogen, to restore toughness and to prevent stress corrosion cracking. However, application of such treatments is very troublesome and disadvantageous in terms of cost and delivery leadtime and it is evident that execution thereof has an adverse effect on product quality.